The present invention is generally related to the field of pulsed electromagnetic energy source systems suitable for material and biological tissue modification processing and removal and is more particularly related to a material removal and modification method and apparatus in which pulsed electromagnetic sources of high ablation-to-deposition depth ratios are operable at pulse repetition rates ranging up to approximately several hundreds of thousands of pulses per second so as to efficiently and precisely remove substantial material volumes while substantially eliminating collateral damage.
The past three decades have brought increased interest in the use of lasers in material processing applications. Early procedures for material processing and cutting involved optical drilling using continuous wave or relatively long pulse (e.g., 50 to 350 xcexcs) lasers such as CO2, ruby and ND:YAG (Neodymium doped Yittrium Aluminum Garnet). These systems, however, required relatively high radiant exposure and resulted in significant alterations to surrounding tissue. As a consequence, lasers could become an effective cutting tool only in areas which did not require high degree of precision or control.
Optical drilling with ER:YAG (Erbium doped YAG) lasers yielded encouraging results in the late 1980s, and has demonstrated its capability to perform as an efficient drill while incurring only relatively low levels of collateral damage to surrounding tissue, provided that no more than one to three pulses per second were applied to the target material. The success of ER:YAG systems, operating in the microsecond pulse duration regime and minimizing thermal damage has also been observed in several areas of applications in material processing and medicine, and can be attributed to the high absorption coefficient of these materials at the particular wavelengths characteristic of the Er:YAG system (2900 nm), when used in combination with the relatively short pulse durations and at low pulse repetition rates.
Laser systems adapted to hard tissue processing, such as dentin and enamel removal in dental applications are disclosed in: 1. Hibst R, Kelly U. Experimental studies of the application of the Er:YAG laser on dental hard substances: I. Measurement of the Ablation Rate. Laser Surgery and Medicine 1989, 9:352-7; and, 2. Keller U, Hibst R. Experimental studies of the application of the Er:YAG laser on dental hard substances: II. Light microscopy and SEM investigations. Lasers in Surgery and Medicine 1989; 9:345-351.)
Both pulsed CO2 and Er:YAG are disclosed in: Walsh, J. T., Flotte, T. J., Anderson, R. R., Deutsch, T. F., xe2x80x9cPulsed CO2 Laser Tissue Ablation: Effect of Tissue Type and Pulse Duration on Thermal Damage,xe2x80x9d Lasers in Surgery and Medicine, Vol. 8, pp. 108-118, 1988; Walsh, J. T., Flotte, T. J., Deutsch, T. F., xe2x80x9cEr:YAG Laser Ablation of Tissue: Effect of Pulse Duration and Tissue Type on Thermal Damage,xe2x80x9d Lasers in Surgery and Medicine, Vol. 9, No. 4, pp. 314, 1989; and Walsh, J. T., Deutsch, T. F., xe2x80x9cEr:YAG Laser Ablation of Tissue: Measurement of Ablation Rates,xe2x80x9d Lasers in Surgery and Medicine, Vol. 9 No. 4, pp. 327, 1989.
A Ho:YSGG laser system is disclosed in Joseph Neev, Kevin Pham, Jon P. Lee, Joel M. White, xe2x80x9cDentin Ablation with Three Infrared Lasers,xe2x80x9d Lasers in Surgery and Medicine, 18:121-128 (1996).
The laser systems disclosed (Er:YSGG, HO:YSGG, and Pulsed CO2) all operate in the IR region of the electromagnetic spectrum and are pulsed in two different regimes: about 250 microsecond pulse durations for the ER:YSGG and HO:YSGG lasers, and about 150 microsecond pulse durations for the CO2 system.
While the disclosed removal rate is in the range of approximately tens of micrometers per pulse, the disclosed laser systems exhibit wavelength dependent absorption and result in high removal rates by operating at pulse energies in excess of 30 millijoules per pulse and often on the order of a few hundreds of xcexcJ per pulse. Enhancing material removal by increasing laser power is, however, accompanied by increased photothermal and photomechanical effects which causes collateral damage in adjacent material. In addition, increasing power leads to plasma de coupling of the beam, e.g., incident laser energy is wasted in heating the ambient in front of the target. High intensity pulses additionally cause very loud acoustic snaps, when the laser pulse interacts with tissue. These snaps or pops include a large high frequency component which is very objectionable to a user or, in the case of a medical application, to a patient. In addition to the psychological impact of such noise, these high frequency snaps are able to cause hearing loss in clinicians when repeated over a period of time.
U.S. Pat. No. 5,342,198, to Vassiliadis, et al. discloses an ER:YAG IR laser system suitable for the removal of dentin in dental applications. The laser produces a pulsed output having a beam with a pulse duration in the range of several tens of picoseconds to about several milliseconds. Although disclosed as being efficient in the removal of dentin and dental enamel, the mechanism by which material removal is effected is not understood. Significantly, however, the only laser systems disclosed as suitable for the process are those which operate at wavelengths (1.5 to 3.5 microns) that have proven to be generally effective for enamel interaction. Thus, the absorption characteristics of the material target are of primary concern to the removal rate. In addition, high energy levels are required to remove enamel and dentin, leading to the problem of thermal damage and acoustic noise.
Additional possibilities for the application of lasers to the field of dentistry in particular, and to hard tissue ablation in general, have been proposed by the use of excimer lasers that emit high intensity pulses of ultraviolet (UV) light.
Several such pulsed UV excimer laser systems, typically with pulse durations in the approximately 1 to 125 nanosecond range are disclosed in:
1. Neev J, Stabholz A., Liaw L. L, Torabinejad M, Fujishige J. T., Ho P. H, Berns M. W., xe2x80x9cScanning Electron Microscopy and Thermal characteristics of Dentin ablated by a short-pulse XeCl Laserxe2x80x9d, Lasers in Surgery and Medicine;
2. Neev J, Liaw L, Raney D, Fujishige J, Ho P, Berns M. Selectivity and efficiency in the ablation of hard Dental tissue with ArF pulsed excimer lasers. Lasers Surgery and Medicine 1991; 11:499-510;
3. Neev J, Raney D, Whalen W, Fujishige J, Ho P, McGrann J, Berns M. Ablation of hard dental tissue with 193 nm pulsed laser radiation: A photophysical study. Spie proceedings, January 1991; and
4. Neev J, Raney D, Whalen W, Fujishige J, Ho P, McGrann J, Berns M. Dentin ablation with two excimer lasers: A comparative study of physical characteristics. Lasers Life Sci 1992; 4(3):1-25. Both the short wavelengths and nanosecond range pulse durations used by excimer lasers contribute to define a different regime of laser-tissue-interaction. Short wavelength ultraviolet photons are energetic enough to directly break chemical bonds in organic molecules. As a consequence, UV excimer lasers can often vaporize a material target with minimal thermal energy transfer to adjacent tissue. The resultant gas (the vaporization product) is ejected away from the target surface, leaving the target relatively free from melt, recast, or other evidence of thermal damage.
Another important characteristic of UV excimer lasers is that materials which are transparent to light in the visible or near infra-red portions of the electromagnetic spectrum often begin to exhibit strong absorption in the UV region of the spectrum. It is well established that the stronger a materials absorption at a particular wavelength, the shallower the penetration achieved by a laser pulse having that wavelength. Thus, in many types of materials, a pulse typically only penetrates to a depth in the range of from about 10 to about 100 micrometers. By simply counting pulses, great precision can be achieved in defining removal depths. In addition, organic tissue is strongly absorbent in the UV wavelengths (193 nm for ArF, for example) therefore allowing the laser-tissue interaction region to be controlled with great precision.
Notwithstanding the relatively damage free material removal characteristics of UV excimer lasers, these systems suffer from several disadvantages which limit their applicability to biological tissue processing. The reports of damage free tissue removal result from evaluations performed on single pulses, or on pulses with a very low repetition rate (typically about 1 to 10 Hertz). Because of the low volumetric removal per pulse of excimer systems (material removed per unit time is poor), efficient material removal can only be accomplished by high pulse repetition rates. However, when the pulse repetition rate exceeds about 3 to 5 Hertz, considerable thermal and mechanical collateral damage is observed. While UV photons are sufficiently energetic to directly break chemical bonds, they are also sufficiently energetic to promote mutagenic effects in tissue irradiated at UV wavelengths, raising concerns about the long term safety and health of a system operator. The scattered light produced by excimer lasers also presents a significant threat to the clinician and/or the patient. Even low intensity scattered radiation, with wavelengths below 300 nanometers, is able to interact with the ambient environment to produce atomic oxygen and other free radicals. These can, in turn, react with the lens and cornea of the eye, producing cataracts, and produce burns on the skin equivalent to sun burns. As a consequence, excimer laser systems have been found to be most suitable for inorganic material processing applications, such as thin coating patterning or dielectric or semiconductor material etching.
In addition, the operational parameters of excimer laser systems are such that material removal remains a wavelength and beam energy dependent process (although weakly dependent on wavelength). Even when pulsed in the tens of nanoseconds pulse duration regime, excimer lasers are configured to deliver energy in the range of from about 10 to about 1000 millijoules per pulse. At the higher energies, excimer lasers suffer from the same problems caused by plasma decoupling and pulse to pulse interaction as IR lasers. Additionally, as pulse energy increases, so too does the intensity of the associated acoustic snap.
Neev et al. (University of California Case No. 95-313-1) U.S. patent application Ser. No. 08/584,522 described a Selective material removal processing Ultra Short Pulse Lasers (USPL) system in combination with a feedback system and with higher pulse repetition rates. This invention is directed to a system for efficient biological tissue removal using ultra short pulses. Such pulse durations are shorter than the characteristics electron-phonon energy transfer time, thus minimizing collateral thermal damage. The method also requires that plasma is formed and decayed so that a thin layer portion of the material is removed. The plasma formation step is then repeated at a pulse repetition rate greater than 10 pulses per second until a sufficient depth of material has been removed with little transfer of thermal or mechanical energy into the remaining material due to the shortness of the pulse duration. The preferred wavelength for that invention is in the range of 200-2500 nm. The laser specified in that patent application is a Chirped Pulse Amplifier (CPA) Solid-state laser.
That patent further specified that the laser system is comprised of a feedback means for analyzing material characteristics in response to interaction between the laser pulses. The envisioned feedback means comprises a spectrograph to evaluate the plasma formed by each pulse. The feedback means is operatively coupled to the laser. The laser operatively responds to the control signal such that the laser ceases operation upon receipt of the control signal. The feedback means also comprises an opticaltomograph which optically evaluates the amount of target material removed by each pulse.
This invention should work well in many applications. Unfortunately, the equipment for the ultrashort pulse duration is very expensive (currently, over $100,000 and often two or three times that amount) and still requires many components and careful maintenance. The systems are also very large and delicate and require large volume for storage and expert maintenance at this stage of the technology. Also the interaction is not very selective nor highly sensitive to the targeted material type but rather ablate most materials. This, in turn, effects some risk of over ablating or removal of unintended structures. The highly interactive nature of the ultrashort pulse process possess additional problems to attempts to deliver the ultrashort pulse beam to the target. Most optical fibers as well as mirror and lenses could easily be damaged if ablation threshold is exceeded (either through narrowing of the beam spot size, an increase in pulse energy, or compression of the pulse duration). Thus ultrashort pulses are hard to deliver through most conventional delivery systems.
An additional problem is that ultrashort pulse lasers are currently achieved principally in the near IR region of the electromagnetic spectrum. This is a highly transparent region for most biotissue material. Consequently, some portion of the radiation propagates linearly into the material and is not confined to the surface. This additional energy propagating into the target may then encounter more absorbing structures (for example the blood vessels in the retina) and will then result in a secondaryxe2x80x94unintendedxe2x80x94ablative interaction, posing risk to the patients or to the material being processed.
U.S. Pat. No. 4,907,586 issued to Bille and Brown for xe2x80x9cMETHOD FOR RESHAPING THE EYExe2x80x9d, disclosed a method for modifying tissue with a quasi-continuous laser beam to change the optical properties of the eye which comprises controllably setting the volumetric power density of the beam and selecting a desired wavelength for the beam. Tissue modification is accomplished by focusing the beam at a preselected start point in the tissue and moving the beam""s focal point in a predetermined manner relative to the start point throughout a specified volume of the tissue or along a specified path in the tissue.
More particularly, the method describes a sequence of uninterrupted emissions of at least one thousand pulses lasting for at least one second. The pulses were specified lasting approximately one picosecond (1 ps) in duration and of less than 30 micro joules (30 xcexcJ).
The invention disclosed in U.S. Pat. No. 4,907,586 should work well for reshaping the eye, but is confined to the region of 1 ps and thus also involves the generation of ultrashort pulses and their relative low thermal and mechanical deposition of energy during the single pulse interaction. This device thus requires the use of expensive ultrashort pulses with all the specified limitations mentioned above. In addition, this invention is limited to relatively low energies of 30 xcexcJ, which require a very tightly focused beam to affect tissue ablation. The invention will thus not work well for larger areas or for high volume removal rates, which are required in many applications, e.g., dentistry, surgery, etc. This invention is also limited with regards to its ability to deliver pulses through optical fibers, hollow waveguides or conventional optics since the very shorted pulses of 1 ps are also very reactive and will interact with most material used as deliver media. Consequently, specialty optics has to be used and conventional lenses and mirrors as well as optical fiber and conventional hollow waveguides cannot be used.
In the present invention, the inventor has recognized that a much wider range of pulse durations of up to approximately several hundred microseconds will allow the thermal diffusion to remain confined to within a distance of only a few micrometer of the ablated crater. Thus, the present invention is concerned with pulses up to several milliseconds long. With a combination of short pulse to pulse separation and with new requirement on both the number and the rate of the incident sequential pulses, the present invention allows large volume removal or volume processing with substantially little damage to surrounding regions of the target.
The present invention thus allows the use of pulse laser systems that are substantially less expensive and in many instances safer and more efficient than those described by other inventions, while achieving unprecedented volume removal rate, high precision, high efficiency and minimal thermal or mechanical collateral damage.
The present invention specifically addresses and alleviates the above mentioned deficiencies associated with the prior art. More particularly, the present invention comprises a method for ablating a material. The method for ablating a material comprises the steps of directing a pulse of energy at the material and so as to permanently modify a quantity of the material. The pulse is specifically configured to increase a ratio of the quantity of the material which is ablated thereby with respect to the quantity of the material which is permanently modified thereby.
Ablating the material with an energy pulse configured specifically configured to increase the ratio of the quantity of the material which is ablated thereby with respect to the quantity of the material which is permanently modified thereby minimizes undesirable permanent modification of the material.
Preferably, at least one characteristic of the material to be ablated is first determined and then a pulse of the directed energy is defined which increases the ratio of the quantity of the material which is ablated thereby with respect to the quantity of the material which is permanently modified thereby. Thus, the characteristic(s) of the material at least particularly define the pulse which is used to ablate the material. For example, the characteristics of the material which may be determined may comprise thermal conductivity, effective electromagnetic energy depth, material energy gapped between valence and conductivity bands, material density or material strength, taken either alone or in combination with one another.
Although the directed energy pulse is described herein as being comprised of laser radiation, those skilled in the art will appreciate that various different types of direct energy, accelerated electrons, accelerated ions, various forms of electromagnetic energy, etc., are likewise suitable. The directed energy may also comprise light from either an LED, a fluorescent lamp, or an incandescent lamp, taken alone or in combination with one another, the directed energy pulse may comprise either coherent or incoherent electromagnetic radiation or any combination thereof.
The characteristic(s) of the material to be ablated may be determined in a variety of different ways, such as by directly sensing the characteristic(s) of the material, by looking up the characteristic(s) of the material in a reference, or by ablating the material with a pulse of the direct energy, determining the approximate quantity of the material ablated and also determines the approximate quantity of the material permanently modified. Thus, one way of determining the desired characteristic(s) of the material to be ablated is by first ablating a quantity of the material and then observing how much of the material is actually ablated versus how much of the material is permanently modified during the ablation process. This determination facilitates adjustment defining the pulse in a desired manner so as to minimize residual heat, thereby minimizing the quantity of the material permanently modified subsequent pulses.
According to a further aspect of the present invention, a plurality of pulses of energy are directed at the material so as to ablate a quantity of the material and so as to permanently modify a quantity of the material. The pulses have a sufficient pulse rate so as to increase the ratio of the quantity of the material which ablated thereby with respect to the quantity of the material which is permanently modified thereby. In this manner, the material is ablated with a plurality of directed energy pulses having a sufficient pulse rate as to minimize undesirable permanent modification of the material.
In a manner similar to that utilized for a single pulse, at least one characteristic(s) of the material to be ablated is determined when utilizing a plurality of pulses. The characteristic(s) of the material to be ablated are then utilized to define the pulse rate of the directed energy so as to again increase the ratio of the quantity of the material which is ablated thereby with respect to the quantity of the material which is permanently modified thereby.
Of course, both a pulse of the directed energy and a pulse rate of the direct energy may be defined by the characteristic(s) of the material to be ablated such that the combination of both the specifically configured pulse and the pulse rate cooperate to increase the ratio of the quantity of the material which will be ablated by the plurality of pulses with respect to the quantity of the material which will be permanently modified thereby.
Thus, according to the present invention, a material is ablated by utilizing a laser. The laser is specifically configured for use with the material so as to cause a substantial quantity of the energy absorbed by the material to subsequently be removed therefrom with the material ejected during ablation. Removing a substantial amount of the energy absorbed by the material minimizes residual energy deposition while ablating, so as to mitigate collateral thermal damage to the material. Ablation of the material is preferably formed at a velocity greater than the thermal energy diffusion through the material so as to remove residual energy from the material.
The material is preferably ablated using a laser having a sufficiently high pulse repetition rate to cause a substantial amount of the energy absorbed by the material to subsequently be removed therefrom with the ejected material. The characteristic(s) of a laser beam pulse are based upon properties of the material so as to provide a depth of the material removed by the pulse which is approximately equal to an electromagnetic deposition depth of the material.
Optionally, the material is ablated utilizing a laser wherein characteristic(s) of the laser beam pulse are based upon properties of the material so as to provide a plasma. The plasma is generated by either multiphoton ionization or thermal ionization. The plasma effects an electromagnetic energy deposition depth which is approximate to a depth of the material removed by the pulse.
Optionally, doping agents are added to the material being ablated. The doping agents cause the laser to provide an electromagnetic energy deposition depth which is approximately equal to the depth of the material removed by the laser.
More particularly, according to the methodology of the present invention high precision, highly controllable, variable rate, material removal is provided by a pulsed electromagnetic radiation beam. The interaction between the pulse electromagnetic radiation beam and the material effects a material removal depth substantially equal to the energy deposition depth within the target material.
The method comprises the steps of providing an electromagnetic radiation source capable of generating an output beam comprised of a sequence of electromagnetic pulses, each pulse having a pulse duration in the range of approximately 1 femtosecond to approximately 10 milliseconds.
The sources operated and beam parameters of the electromagnetic radiation output beam are manipulated so that the electromagnetic pulse""s power densities within the region targeted for energy deposition is in the range of is approximately 107 W/cm3 to approximately 1018 W/cm3 and is larger than the power density threshold for material ablation.
Thus, the material is ablated with electromagnetic energy from the source so that a substantial portion of the deposited electromagnetic energy is removed from the target material with an ejected portion of the material.
Ablation of the material is repeated at a pulse repetition rate greater than approximately 0.1 pulses per second so that a substantial portion of the cumulative residual thermal energy left in the material by the electromagnetic energy is removed by the cumulative ablation. The pulse repetition rate is preferably smaller than approximately 500,000 pulses per second. This process continues until a sufficient depth of the material has been removed.
The electromagnetic beam""s energy deposition depth within the material defines a volume so that the power density within the volume is greater than the threshold power density for material ablation.
The pulsed electromagnetic radiation source preferably produces an output beam having a wave length in the range of approximately 10 nanometers to approximately 50 micrometers.
Each pulse of the pulsed force preferably has an energy in the range of approximately 0.001 microjoule to approximately 50 Joule. The output beam preferably has a diameter at the target material such that the target material experiences an energy fluence in the range of approximately 0.001 Joule per square centimeter to approximately 100 Joule per square centimeter.
The pulsed beam preferably exhibits a material removal rate in the range of approximately 0.01 micrometers to approximately 100,000 micrometers per pulse. The removal rate is preferably substantially constant.
According to a further aspect of the present invention, precise, highly controlled, variable rate material removal is provided by a pulsed electromagnetic radiation beam. A source capable of generating an output beam comprised of a sequence of electromagnetic pulses is provided. Preferably, each electromagnetic pulse has a pulsed duration in the range of approximately 1 femtosecond to approximately 10 milliseconds.
The source is operated and the beam parameters manipulated so that the electromagnetic pulse""s power densities within the region targeted for energy deposition is in the range of approximately 108 W/cm3 to approximately 1018 W/cm3 and is larger than the power density threshold for plasma formation.
The formed plasma is allowed to decay such that a layer of the material is removed. The removed layer of material carries with it a substantial portion of the deposited electromagnetic energy from the target regions.
The pulse source is operated so that once a critical electron density is reached within the formed plasma, the formed plasma substantially presents excess pulse energy form directly reaching the material and so that the formed plasma prevents excess pulse energy from substantially increasing the electromagnetic energy deposition depth and the depth of the material removed by ablation.
The pulse source is operated at a pulse repetition rate greater than approximately 0.1 pulses per second and less than approximately 500,000 pulses per second until a sufficient depth of material has been removed.
According to a preferred embodiment of the present invention, the laser beam defines a spot on the target characterized in that fluence within the beam spot is greater than the threshold fluence for plasma formation.
The plasma formation substantially prevents deep energy deposition in the material so that a substantial portion of the electromagnetic energy deposited in the material is removed with the material ejected.
The pulsed electromagnetic energy source preferably produces an output beam having a wavelength in the range of from approximately 10 nanometers to approximately 15 micrometers. Each pulse of the pulsed energy source preferably has an energy in the range of approximately 0.001 microjoule to approximately 100 Joule. The output beam preferably has a diameter at the material target such that the material experiences an energy fluence in the range of approximately 0.001 to approximately 100 Joule per square centimeter.
The pulse laser beam preferably exhibits a material removal rate in the range of approximately 0.01 to approximately 10 micrometers per pulse. The removal rate is preferably substantially constant without regard to material chromophore, material hardness or material state.
Optionally, the target material is substantially transparent to the linear propagation of the electromagnetic pulses and the beam is focused below the surface of the target material so that the beam intensity exceeds the plasma formation threshold only at approximately the point of focus and the material is substantially removed at that desired point below the surface. In this manner, material can be ablated in a manner which forms caverns or hollow volumes within the material. Thus, cavities having various different desired shapes may be so formed. This may be accomplished using either a single pulse, or a plurality of pulses, as desired.
Alternatively, the method for controlled variable rate material removal by a pulsed electromagnetic radiation beam comprises providing a source capable of generating an output beam comprised of a sequence of electromagnetic pulses, wherein each electromagnetic pulse has a pulse duration in the range of about 1 femtosecond to about 10 milliseconds and also comprises operating the pulse source and manipulating the beam parameters so that the electromagnetic pulses"" peak intensity is in the range of approximately 10 W/cm2 to approximately 1016 W/cm2 and adding to the target material absorption or scattering centers, defects, highly absorbing or highly scattering components, so that the electromagnetic radiation penetration depth is reduced and/or plasma is formed.
Preferably, the electromagnetic energies absorbed by the material to complete the material disintegration and explosive ejection of the targeted material deposition volume, so that substantially most of the deposited energy is removed from the target material with the ejected portion of the material. The pulse source is preferably operated at a pulse repetition rate greater than approximately 0.1 pulses per second smaller than approximately 500,000 pulses per second until a sufficient depth of material has been removed.
Further, according to the preferred embodiment of the present invention, plasma is formed and expanded, substantially preventing excess pulse energy from directly reaching the material and so that excess pulse energy does not substantially effect ablation depth. The plasma is preferably allowed to decay such that a layer of the material is removed and substantially most of the material radiation pulse energy deposited in the material is removed with the layer. The source is operated at a pulse repetition rate greater than approximately 0.1 pulses per second and less than approximately 500,000 pulses per second until a sufficient depth of material has been removed.
In a further alternative method for controlled, variable rate material modification, a pulsed electromagnetic radiation beam is provided by providing a source capable of generating and output beam comprised of a sequence of electromagnetic pulses, each electromagnetic pulse having a pulse duration in the range of approximately 1 femtosecond to approximately 100 milliseconds. The pulse source is operated and the beam parameter is manipulated so that the deposited volumetric power density within the targeted material is greater than the threshold power density for material modification, such that control of the power density is achieved by varying either one or more of the following parameters: the beam spot size at the target location, the duration of the electromagnetic pulse emissions, the energy of the electromagnetic pulse emissions, the wavelength of the electromagnetic pulse emissions, or by spatially and temporally varying the absorption and/or scattering characteristics of the material at the target region. The interaction energy transients caused by the electromagnetic radiation pulse are allowed to substantially decay such that material modification is effected. Such material modification preferably includes one or more of the of the following alterations: Chemical and physical changes, changes to visco elastic properties, changes to optical or thermal properties, chemical and physical breakdown disintegration, ablation, melting, or vaporization.
The pulse source is preferably operated at a pulse repetition rate greater than approximately 0.1 pulses per second until a sufficient volume of the material has been modified.
The target material is preferably substantially transparent to linear beam propagation and the threshold volumetric power density is achieved at a desired target location below the surface and within the material volume. Again, scattering and/or absorption centers, defects, or highly absorbing components are added to the target material with spatial and/or temporal selectivity to specific, predetermined locations within the target material.
The pulse beam preferably exhibits a material modification rate in the range of approximately 0.013 cubic micrometers per pulse to approximately 100,0003 cubic micrometers per pulse. The material modification rate is preferably substantially constant, depending substantially on the volumetric power density threshold characteristics of the material and on the target beam characteristics thereof.
Thus, a method and apparatus is disclosed for fast, precise and damage free material processing and modification using a high pulse repetition rate electromagnetic energy source. The pulsed interaction uses a parameter regime which minimizes residual energy deposition while ablating. Advantageously, removal of cumulative pulse train residual energy is maximized through the rapid progression of the ablation front which moves faster than the thermal energy diffusion. Removal of residual energy thus minimizes collateral thermal and mechanical damage in material processing and also minimizes pain and suffering during surgical procedures. The operating parameters for the system are achieved through the selection of material properties and beam characteristics which ensure localization of incoming electromagnetic energy into a deposition zone comparable in its depth and lateral dimensions to the depth and lateral dimensions of the volume of ablated material. The disclosed method identifies either high linear absorption or plasma-mediated interactions, or a combination of the two, as potential avenues for fulfilling the deposition depthxe2x80x94ablation depth nearxe2x80x94parity requirements which enable high pulse repetition rate operation and thus ensure rapid material removal. The disclosed apparatus then describes a variable repetition rate system which allows highly adjustable material removal rates ranging from very rapid to very slow. A set of possible energy delivery and collection systems is then disclosed which allow highly accurate delivery, monitoring, feedback, control, and automation for extreme precision and unprecedented accuracy that can be offered simultaneously with the rapid rate of operation. Finally, a method and apparatus are disclosed for fast, efficient, precise and damage-free material modification, utilizing the threshold nature of plasma-mediated interaction and/or selectively induced, high absorption regions. Making use of the same apparatus with the option to spatially and/or temporally control the addition of xe2x80x9cdoping agentsxe2x80x9d, to induce selective power deposition, precise and highly localized material removal and/or modification can be induced at any desired location within the three-dimensional space of the target region while substantially sparing adjacent regions of the target material from any collateral damage.
Further, a method for high precision, highly controllable, variable rate, material removal by a continuously emitting, continuous wave (CW) beam of electromagnetic radiation is provided. The interaction between the electromagnetic beam and the material is such that a material removal depth is approximately equal to an energy deposition depth within the target material. The method comprises the steps of: providing a source capable of generating an output beam comprised of continuously emitted electromagnetic radiation; and redistributing the beam in time and space to form at least one modified beam. This is accomplished by repeatedly selecting a portion of the beam in time and redirecting that portion of the beam so as to define a series of pulses. In this manner, a plurality of time sequential segments of the beam are redirected, preferably through an optical fiber. Thus, a short segment of the CW beam, enough to provide the desired pulse, is diverted from the remainder of the beam. Other portions of the beam may similarly be redirected such that the entire beam is utilized.
In effect, the beam is time sliced, such that for a given duration, a portion of the beam is directed to one optical fiber, and then for another period of time the optical beam is directed to another optical fiber. This process is repeated for a desired number of optical fibers (optimally until the entire beam is utilized) and after the beam has been directed to each of the optical fibers, the process repeats. In this manner, the beam is sequentially directed from one optical fiber to another, so as to define the desired number of separated pulsed beams. Each of the optical fibers directs the beam to a desired location where material ablation is to occur. Generally, each portion of the beam will be directed such that it is incident upon adjacent (optionally overlapping) areas of the material to be ablated such that the different portions of the beam cooperate so as to effect material ablation in an efficient and effective manner which minimizes undesirable modification of adjacent material due to overheating.
Thus, the beam is redirected so that either a single or multiple beams are formed and such that their energy distribution at any given location on the target material forms a sequence of electromagnetic pulses. Each electromagnetic pulse preferably has a duration between approximately 1 femtosecond and approximately 10 milliseconds.
Thus, the beam is modified such that the original beam is re-configured into a new single or multiple beams. In this manner, the energy of the original beam is utilized after having been redistributed in both time and space.
The source of electromagnetic energy is operated and the beam parameters are manipulated so that the electromagnetic pulse""s power densities within the region targeted for modification are between approximately 104 W/cm3 and approximately 1018 W/cm3 and are larger than the power density threshold for material ablation. The electromagnetic energy absorbed by the material is allowed to complete the material ablation process, so that substantially most of the deposited electromagnetic energy is removed from the target material with an ejected portion of the material, as discussed in detail above.
Such electromagnetic energy absorption is repeated, as desired so that ablation and energy removal occurs at a pulse repetition rate greater than 0.1 pulses per second, such that substantially most of the cumulative residual thermal energy left in the material by a pulse train is removed by the cumulative ablation. Thus, ablation is performed at a pulse repetition rate less than approximately 500,000 pulses per second until a sufficient depth of material has been removed with substantially no transfer of thermal or mechanical energy into the main material and substantially no collateral damage thereto.
The step of redistributing the beam preferably comprises deflecting sequential portions of the beam and redirecting them to a distinguished, separate locations upon the target material, so that the net affect at each location is that of a sequence of pulses of a specified or desired duration and a specified or desired pulse repetition rate.
In this manner, the switching device redirects sequential portions of the beam to separate locations so that the net affect at each location is that of a sequence of pulses of specific duration and the step of redistributing the beam comprises directing the beam to a device selected from a device such as a rapidly rotating mirror or other optical means for directing electromagnetic radiation, a Kerr cell, a Pockels cell, and acousto optic modulator, an electro optic modulator, or any other electro-optical, electrical, magnetic, or electromagnetic means for redirecting light.
The switching device sequentially redirects the original beam energy into at least one optical guiding device such as an optical fiber or a hollow wave guide light conductors. The optical guiding device then redistributes the beam to separate locations on the target material.
Preferably, the output of the single or multiple optical fibers or hollow wave guides is performed so as to focus the energy to a spot size such that the power density within the volume target is for material removal for modification is greater than the threshold power density for material ablation or the desired modification.
The step of redistributing the beam preferably comprises redirecting the original beam energy into a single or multiple lenses or other optical focusing devices and then allowing the newly redistributing beams to propagate to separate locations on the target material.
The pulse electromagnetic radiation source produces an output beam having a wavelength in the range of 10 nanometers to 50 micrometers.
The continuously emitting beam source preferably has an average power in the range of approximately 0.0001 Watt to approximately 500 KWatts. The output beam preferably has a diameter at the target material such that the target material experiences a power per unit area in the range of approximately 1 Watt per square centimeter to approximately 1014 Watts per square centimeter.
The pulse beam is preferably configured to provide a material removal rate in the range of approximately 0.01 micrometer to approximately 10,000 micrometers per pulse. The material removal rate per pulse is preferably substantially constant.
Each of the redistributed beams comprises a sequence of electromagnetic pulses, each pulse preferably having a pulse duration in the range of approximately 1 femtosecond to approximately 10 milliseconds and has a pulse repetition rate greater than approximately 0.1 pulses per second and less than approximately 100,000 pulses per second.
The redistributed beam preferably comprise a sequence of electromagnetic pulses which is directed to a target location adjacent one another. In this manner, the beams cooperate with one another so as to remove at least some thermal energy generated by preceding pulses and these adjacent beams.
The step of redistributing the beam preferably further comprises changing the beam wavelength via a device such as an optical parametric oscillator, and optical parametric amplifier, or a non-linear frequency converting crystal such as KTP or KDP. In this manner, the frequency of the beam is doubled, tripled, quadrupled, etc., as desired.
According to the preferred embodiment of the present invention, a device for high precision, highly controllable, variable rate, material removal by a continuously emitting, continuous wave (CW) beam of electromagnetic radiation wherein the interaction between the electromagnet beam and the material being characterized by a material removal depth which is substantially comparable to the energy deposition depth within the target material. The device preferably comprises an energy radiating device for providing a continuously emitted electromagnetic energy beam and a first controller for redistributing the energy beam into at least redistributed beam which4is redistributed in space and time, i.e., time sliced.
A controller redirects the redistributed beams so that the energy distribution at any given location on the target material forms a sequence of electromagnetic pulses, each electromagnetic pulse having a pulse duration in the range of approximately 1 femtosecond to approximately 10 milliseconds. The device and its controllers are preferably operated such that the output electromagnetic power densities within the region targeted for modification is in the range of approximately 104 W/cm3 to about 1018 W/cm3 and is larger than the power density threshold for material ablation. The electromagnetic energy absorption is permitted to continue until the desired material abrasion is complete, so that substantially most of the deposited electromagnetic energy is removed from the target material with the ejected portion of the material.
Electromagnetic energy absorption is repeated at a pulse repetition rate greater than 0.1 pulses per second such that most of the cumulative residual thermal energy which remains in the material due to the pulse train is removed by the cumulative ablation. The pulsed repetition rate is thus preferably smaller than approximately 100,000 pulses per second and continues until a sufficient depth of material has been removed with substantially no transfer of thermal or mechanical energy into the remaining material and substantially no collateral damage thereto.
The first controller preferably comprises a switching device which deflects sequential portions of the beam and redirects them to separate locations such that the net effect at each location is that of a sequence of pulses of specific duration and specific pulse rate. In this manner, rather than illuminating the entire portion of material to be ablated, different portions thereof are illuminated sequentially, thereby enhancing the ability of the cooperating beams to remove heat therefrom.
The first controller preferably comprises a switching device such as a rapidly rotating mirror, a Kerr cell, a Pockels cell, and acousto-optic modulator, an electro-optic modulator, or other electro-optical, electrical, magnetic, or electromagnetic means for redirecting light.
These, as well as other advantages of the present invention will be more apparent from the following description and drawings. It is understood that changes in the specific structure shown and described may be made within the scope of the claims without departing from the spirit of the invention. The method for ablating a material with a directed energy pulse, such as that of a laser, includes directing a pulse of energy at the material so as to ablate a quantity of the material and so as to permanently modify a quantity of the material, the pulse being configured to increase a ratio of the quantity of the material which is ablated thereby with respect to a quantity of the material which is permanently modified thereby. Alternatively, a plurality of pulses of energy are directed at the material so as to ablate a quantity of the material and so as to permanently modify a quantity of the material, the pulses having a sufficient pulse rate as to increase a ratio of the quantity of the material which is ablated thereby with respect to the quantity of the material which is permanently modified thereby. Ablating the material with an energy pulse or with a plurality of energy pulses configured so as to increase the ratio of the quantity of the material which is ablated thereby with respect to the quantity of the material which is permanently modified thereby minimizes undesirable permanent modification of the material.